Hypoxia and Equipment Failure

Hypoxia and Equipment FailureApril 25, 2011 Case Stem:A 70 year old man is to undergo cystoscopy and transurethral resection of a bladder tumor under general anesthesia through an LMA. He gave a history of mild asthma and used an albuterol inhaler when necessary. Breathing room air (FiO2 = 0.21), his pulse oximeter saturation reading (SpO2) was 94%.What is Hypoxia and Hypoxemia?HypoxiaReduction of oxygen supply to tissue below physiologic levels.Decreased oxygen tension (PO2) inside the body at tissue level or outside the body (hypoxic gas mixture)HypoxemiaDeficient oxygenation of blood.Decreased oxygen tension in the arterial blood (PaO2)Is PaO2 related to age?Yes.Age-dependent decrease in PaO2.Marshall and Whyche equationMean PaO2 (mmHg) = 102-0.33(age in years)Sorbini et al. found PaO2 decreased from about 95 mmHg at 20 years of age to 73 at 75 years (about 4-5 mmHg per decade)

Was this patient hypoxemic?No.Hypoxemia is considered to exist when the PaO2 is less than 60 mmHg which is equivalent to a hemoglobin O2 saturation of 90%

Using the Marshall Whyche equation102-0.33(70) = 79 mmHgWhat is a pulse oximeter and what is a hemoximeter?Pulse OximeterNoninvasive device that provides an estimate (SpO2) of the arterial hemoglobin saturation with oxygen.Uses patient body part as in vivo cuvette through which 2 different wavelengths of light are transmitted.HemoximeterUsed to analyze an arterial blood sample.Laboratory cooximeter that uses six or more different wavelengths of light to measure total hemoglobin, oxygenated hemoglobin, deoxygenated hemoglobin, methemoglobin, carboxyhemoglobin and other aberrations. How does a pulse oximeter work?Pulse oximeterLight-emitting diodes transmit red light at wavelengths 660 nm and infrared light at 960 nm through the probe site. Light is sensed by a single photodetectorRatio of absorbances (660/990 nm) is related to hemoglobin O2 saturation (Spectophotometry)Plethysmography detection of pulsatile flow (as blood pulses, absorbance increases)What affects the accuracy of a two-wavelength pulse ox?Pulse oximeterPatient movement (shivering, peripheral nerve stimulation, twitching)Presence of intense ambient lightElectrocautery useAdministration of IV dyes with absorbance peaks at 66o nm (methylene blue)DyshemoglobinemiasNail polishPoor pulsatile flow at probe site (hypotension, Raynauds)Venous pulsations (tricuspid regurg)How do methemoglobin and carboxyhemoglobin affect SpO2 readings?MethemoglobinIron in heme moiety is oxidized (dapsone, benzocaine, nitric oxide, prilocaine) to Fe3+ state rather than Fe2+ state.Cannot carry O2Shows similar absorbances at 660 and 940 nm (SpO2 tends toward 85%)Overestimates the fractional saturation and underestimates the functional saturationCarboxyhemoglobinCO + Hb has similar absorbance to HbO2 at 660 nm, but very low absorbance at 94o nm. SpO2 overestimates fractional saturation and underestimates functional saturation.SpO2 will appear in the 90sHemoximeter required to determine true O2 satWhat is a capnometer and capnography?CapnographyMost use infrared spectroscopy to measure PCO2A built in barometer measures barometric pressure so that CO2 can be displayed as a percentage.Gold standard for establishing presence of ventilation.What is meant by end-tidal CO2 concentration?End-tidal CO2Tension of CO2 in the exhaled gas at end of exhalation.Represents the CO2 tension in the alveolar gas (PACO2)Does not account for dead space ventilationPresence of CO2 depends onProduction of CO2 by the tissuesCO and pulmonary blood flow to carry CO2VentilationCapnogramPhase I Expiratory baselinePhase II Expiratory upstrokePhase III Expiratory plateauHorizontal in healthy lungsUpward Slope with obstructive airway diseaseMaximum expired CO2 is considered the end-tidal angle slope between II and III (increase in acute bronchospasm)Phase IV Inspiratory downstrokeCapnograms AberrationsElevated baseline CO2Capnometer not properly calibrated to zeroDelivery of CO2 to breathing system through fresh gas inflowIncompetent unidirectional valvesFailure of CO2 absorber (channeling, exhaustion, bypass)Prolonged expiratory plauteau and expiratory upstrokeMechanical obstruction to exhalationCOPDBronchospasmDips in expiratory plateauSpontaneous ventilation effortsCardiogenic oscillationsVentilator pressure relief valve pertubationsElevated expiratory plateauIncorrect calibrationIncreased CO2 production / deliveryLaparoscopic CO2 gas insufflationDecreased CO2 removalHypoventilationLeakDecreased expiratory plateauIncorrect calibrationAir leak into gas sampling systemHyperventilationDecreased CO2 production (hypothermia)Increased arterial-alveolar CO2 gradient (VQ mismatch / pulmonary embolus)Prolonged inspiratory downstroke and raised baselineIncompetent or missing inspiratory unidirectional valveInspiratory obstruction to gas flow (kinked tube)What is the A-a difference in CO2?A-a gradientMeasure of alveolar dead space ventilation2.5cc / Kg = volume of anatomic dead space(PaCO2 PETCO2) / PaCO2 = Ratio of dead space to tidal volumAlveolar dead space increased by ventilation in excess of perfusion or decrease in perfusion (shunt has minimal effect)PaCO2-PACO2 nl 3-5 mmHgImportant safety features on anesthesia machineSafety FeaturesPin index (cylinder) and diameter index (pipeline) safety systemsFail-safe valve pressure sensitive device that interrupts flow of all hypoxic gases on the machine to their flow control valves if the supply pressure of O2 in the high pressure system falls below a threshold (between 12-20 psig)O2 supply failure alarm pressure below 30 psigO2 flow control knob fluted and on the rightKey-fill systems for vaporizersPop-off (pressure relief) valveSafety FeaturesGas flow proportioning ensure minimum O2 of 25% when N2O is usedVaporizer interlock systemWhat are sites for gas leakage?Gas LeakageBreathing systemPartially deflated tracheal tube cuffDisconnection of sidestream gas analyzerHumidifiersBagLow-pressure machine componentsCracked rotameter flow tubesIncorrectly mounted vaporizersVaporizer leak around agent filling deviceFracture in gas pipingMachine Check for LeaksDragerCircle breathing system tubing removedInsp and exp limb connected by tubingResevoir bag removed and replaced with test terminal with sphygmomanometer bulbPressurize with bulb to 50 cm H2O pressure should not decrease by 20 in 30 secTest with vaporizers onDatex-OhmedaOne-way outlet check valve at the common gas outletConnect bulb and squeeze, should not refill in 30 secPreop Equipment CheckStep 1 Emergency Ventilation EquipmentStep 2 Check O2 Cylinder supplyStep 3 Central pipeline supplyStep 4 Low-pressure system check (flow control valves and vaporizer status)Step 5 Leak check of low-pressure systemStep 6 Turn on machine master switch and other electrical equipmentStep 7 Test flowmetersStep 8 Adjust / Check scavenging system (test pop-off)Step 9 Calibrate O2 monitorStep 10 Check initial status of breathing system (circuit, CO2 absorbent)Step 11 Leak check of breathing systemStep 12 Test ventilation system (connect resevoir bag to Y-piece)Step 13 Check, calibrate, and set alarm limitsStep 14 Check final status of machineWhat emergency equipment should be readily available?Emergency EquipmentBack-up ventilation equipmentEmergency airway equipmentCricothyroid kit / Difficult airway cartWorking flashlightBackup batteryO2 tank and regulatorMalignant hyperthermia cartCode cartFire extinguisherWhat are goals of premedication?PremedicationAnxiolysisMinimization of gastric volume and acidityAntibiotic prophylaxisAntisialagogue effectStandard ASA MonitorsStandard ASA MonitorsStandard I Qualified anesthesia personnel shall be present in the room throughout the conduct of all general, regional, and monitored anesthetic care.

Standard II During all anesthetics, the patients oxygenation, ventilation, circulation, and temperature shall be continually evaluated.Standard ASA MonitorsOxygen analyzer with low O2 alarmQuantitative method of blood oxygenation (pulse ox)Ventilation evaluation (chest rise, auscultation)Correct positioning of airway devicesEnd-tidal CO2 with airway devicesVentilator disconnection alarmECG, BP, HR (every 5 min for the latter 2)Body temperature (if perturbations are expected)Inhalation mask induction, LMA placed. Sats fall to 81%. Patient develops respiratory distress. LMA is intact. Now what?InterventionsAssess for airway obstruction, bilateral breath sounds, quality of breath soundsCheck FiO2, ETCO2, HR, BP, SpO2Bladder intake and outputIV fluid intakeIncrease FiO2 to 100%Consider assisted ventilationIf no improvement, tracheal intubation and PPVPatient intubated. Squeezing the bag results in failed ventilation and a leak. Where is it?Common Leak SitesIncomplete tracheal cuff sealElbowEnd-tidal monitoring connectionsInspiratory and expiratory hosesUnidirectional valves, Pop-off valve, resevoir bag, bellows, absorber, vaporizers, flowmeters, scavenging system

What are the criteria for extubation?Extubation CriteriaGlobal criteriaReturn of consciousnessDemonstration of ability to protect airwayAdequate reversal of NM blockadeAbsence of hypothermiaPresence of nl metabolic milieuRespiratory criteriaVital capacity > 15ml per kgNIF < -20 cm H2OSpO2 > 90% on FiO2 100 Patient will probably fail extubationIf develops diaphoresis, agitation, tachycardia, bradycardia, HTN, hypotension failed trialWhat are causes of postop hypoxemia?Postop HypoxemiaPhysiologicallyLow FiO2HypoventilationVQ mismatchShuntPathologicallyAirway obstructionAtelectasisR Mainstem intubationAspirationPulmonary edemaPulmonary embolusWhat is the difference between shunt, VQ mismatch, and dead space?ShuntPerfused but not ventilatedPaO2 will not rise with increased FiO2 once shunt fraction approaches 30%Dead SpaceVentilated but not perfusedFailure to maintain normal PaCO2 despite increased MV (tv x rr)What is the differential diagnosis of pulmonary edema?Pulmonary EdemaConditionPathophysiologyCongestive Heart Failure

Negative Pressure Pulmonary Edema

Acute Lung Injury and ARDS filling pressure, CO

outside-inside pressure gradient

PermeabilityARDS Mechanical VentilationTV < 6ml / kgPIP < 35 cm H2OConsider PEEP of 10 cm H2O